The present invention relates to signal-source position-determining systems and, in particular, to interferometer systems that employ a multi-lambda, longitudinal antenna array which inherently produces position ambiguities that must be resolved.
The invention will be described primarily by considering its use or applicability in so-called `Sonobuoys Reference Systems` of the type presently used to resolve airborne ASW problems in which an aircraft flying over the ocean is attempting to detect an invisible submarine operating below the water surface. One of the most common sensors used in such exercises is provided by sonobuoys capable of detecting sounds from underwater targets and relaying them to the aircraft by way of its radio transmitter. Half of the ASW problem thus involves the location of the underwater target relative to a group of sonobuoys. The other half, with which the present invention is directly concerned, involves the location of the sonobuoys relative to the aircraft. Obviously, the accurate location of the sonobuoy relative to the aircraft is needed to enable location of the invisible target. The Sonobuoy Reference System (SRS) can be considered as navigational equipment since its primary purpose is to locate the position of an aircraft relative to a field of drifting sonobuoys. However, in a broader sense, the present invention can be considered as relating generally to the job of locating a signal-generating target of any type or utility. For emphasis, it again is to be noted that the descriptive use of the SRS example is illustrative rather than restrictive.
Accurate, fast, SRS systems are needed and considerable efforts have been devoted to their development. The need becomes apparent when it is recognized that the original splashpoint of the sonobuoy rarely is known accurately. Also, the sonobuoys may drift at rates up to several knots throughout their operating life. A simple solution to the problem can be achieved if the aircraft can be tactically maneuvered so as to fly directly over the buoy at a low altitude. In some situations, this technique, known as an OTPI (on top position indication) technique, can be employed. However, the accuracy of visual OTPI is an inverse function of altitude and, as will be recognized, it also is highly dependent upon the observer's skill and experience. Because of such difficulties, more sophisticated and useful SRS systems incorporate angle-measuring equipment (AME) capable of measuring the angle from the axis of the aircraft to the sonobuoy at, for example, t.sub.1 and again at t.sub.2. By process of triangulation, the location of the aircraft relative to the sonobuoy can be determined. Such a process can be performed at any operating altitude and also it can be used with sonobuoys that are offset from the flight path.
The system of the present invention is of the type which employs the angle measuring equipment and, more specifically, angle measuring equipment which operates on the principle of an interferometer. Characteristically, interferometer systems used for this purpose generally employ at least one pair of antennas mounted on the aircraft and spaced one from the other a particular longitudinal distance defined as their baseline length. Obviously, if the longitudinal baseline of this antenna array is normal to the direction of arrival of a C-W signal from a sonobuoy target, then both antennas will receive the signal at the same time. Consequently, the signals arriving at the antennas will be in phase. However, if the sonobuoy is not located on the normal to the baseline, the signal will arrive at one of the antennas ahead of the others adn there will be a phase difference. By measuring this phase difference (.phi.) it is possible to determine the angular displacement of the sonobuoy.
A difficulty experienced with the use of such systems is that if the baseline of the array is greater than one half the wavelength of the signal source, the phase measurements of the interferometer can be ambiguous because any single phase measurement can represent more than one angle of arrival. The reason for the ambiguity is because the phase detector of the system cannot determine how many cycles have occurred between the signal's time of arrival at the two antennas. For example, a phase difference of thirty degrees will appear to be the same as the phase difference of 30.degree. plus 360.degree. at the output of the detector. This ambiguity is present in all arrays having a baseline longer than one half the wavelength of the signal. Such arrays usually are referred to as multi-lambda (n.lambda.) arrays, the term lambda representing the wavelength of the signal. Such ambiguity, however, is not present in a one half lambda baseline because, in this instance, only 180 electrical degrees separate the arrays and there is no possibility of two angular directions providing the same phase measurement.
Although the foregoing considerations obviously point to the use of the shorter baseline arrays, there is a further problem in that the shorter arrays provide relatively inaccurate results. Consequently, the situation is one in which the shorter arrays are unambiguous but relatively inaccurate whereas the longer arrays are relatively accurate but ambiguous.
This particular difficulty, of course, has been recognized in the implementation of SRS systems presently in use and, as would be expected, various techniques are used in an effort to remove the ambiguity while retaining the accuracy. One such technique employs multi-array configurations including both one half lambda and multi-lambda arrays as well as other arrays intermediate in length between these two. Ambiguities in the multi-lambda baselines are removed by sequentially processing the signal angle of arrival data to progressively remove the ambiguity from the shorter to the longer baselines. Usually, the ambiguity of the intermediate baseline arrays is removed from the one half lambda array data and the ambiguity of the long baseline array is subsequently removed from the intermediate data. In a more recent development (Lockheed-Cubic SRS System) the approach differs to the extent that a priori sonobuoy position data is utilized in the ambiguity removal of various multi-lambda arrays. When the sonobuoy position uncertainty is too great, a one half lambda AOA data is utilized until such time as the sonobuoy position estimate is improved sufficiently to permit the ambiguity to be resolved in the intermediate array data. Sequential processing of the intermediate array data subsequently improves the sonobuoy position estimate and provides a capability for resolving the ambiguities in the longer baseline data. As will be appreciated, the sequential processing of the longer baseline data provides the required accuracy in the sonobuoy position estimate.
One problem with these multi-array systems is that the ambiguity removal of data progressively from the shorter to the longer baselines is not always successful because of the excessive multipath errors and incorrectly resolved data which is generated and fed into the position solution algorithm which then yields an incorrect result. In the Lockheed-Cubic approach, longer baseline arrays can be utilized only when the sonobuoy position is known accurately enough through sonobuoy position estimates computed from sequential processing of the shorter and intermediate baseline data. Thus, one of the drawbacks to this approach is that the full potential of the angle of arrival data accuracy is not utilized immediately. Consequently, the time element involved in the sequential processing has undesirable tactical implications. Further, the success of the ambiguity removal remains somewhat sensitive to large multipath errors.
Another multi-array difficulty is the relative complexity, weight and size of its equipment or components. For example, one SRS multi-array system requires four longitudinal arrays as well as six transverse arrays the antennas of which feed into an SRS receiver-processor unit that includes an antenna switching matrix, an AME receiver, a computer interface unit and other electronic components such as local oscillator and calibration units. Obviously, it is desirable to provide a system in which the hardware requirements can be greatly reduced. Of equal importance, it is desirable to provide a system having the capability of producing relatively instantaneous results in contrast to the time-consuming, sequential processing required in the multi-array systems.